This invention relates in general to mills for rolling metal products such as strands and strips. More specifically, it relates to a roll design particularly useful in rolling a hot metallic strand into a strip of well defined dimensions and good quality that also controls the lateral location of the product on the working rolls.
A wide variety of mill stands are known for hot and cold rolling metals. Where there is a large separating force between the working rolls, whether due to a large reduction and/or to the nature of the material being rolled, there are a number of inherent design problems. One is that the rolls work against a separation force that is sufficiently large to bend or even to deform the rolls depending on the diameter, length and material of the roll as well as the nature of the material, its temperature, and the reduction ratio. The diameter of the roll is also important because for a given "bite" (thickness reduction of the product entering the mill) the "bite ratio" (roll diameter over bite) is an important factor in determining when slippage will occur between the rolls and the product. As low a ratio as possible is desired to minimize roll size (and therefore roll cost) and/or maximize bite. Typical bite ratios for mills currently in use are in the range of 50:1 to 100:1. Another consideration is that larger diameter rolls produce a greater spread, however, the attendant separation force is also larger. Ideally the roll design should produce the desired spread with the minimal separation force.
Heretofore, in order to deal with large separation forces (e.g. in excess of 100,000 lbs/stand), it has been necessary to use a four high mill, that is, one with two working rolls and two "back up" rolls that provide mechanical support for the working rolls. Such mills are also characterized by a quite heavy, expensive frame that can accommodate all four rolls and resist the large forces generated by the rolling. U.S. Pat. Nos. 3,103,138; 3,391,557; 3,550,413 and 3,568,484 are exemplary of such four high mills stands. While certain mills produce a high reduction by passing the product through the mill multiple times, this is not possible in a continuous, on-line casting and rolling operation. (For example, rolling operations with twenty passes are not uncommon.)
Frequently the gap of the mill is adjustable to produce an output strip that has a uniform gauge even though the gauge, temperature or metallurgical qualities of the input product may vary. The aforementioned patents, for example, describe arrangements for varying the gap. More sophisticated systems produce a control signal that adjusts the gap in response to a sensed deviation in the gauge of the rolled product from a preset value. Many systems use hydraulic cylinders that act either with or against the separation force to provide this adjustment. The hydraulic system, however, has heretofore been a significant complicating factor when the rolls must be replaced, whether due to ordinary wear, damage or to accommodate a different product run. U.S. Pat. No. 3,864,955, for example, discusses the importance of roll changes and describes an arrangement which attempts to facilitate roll replacement. U.S. Pat. No. 3,323,344 discloses another arrangement. However, known systems, for conventional four high mills require hours to replace rolls. This time represents a significant loss of productivity for the rolling operation. Where the product being fed to the mill is continuously cast, the roll change will shut down the entire production line.
The torque and speed of rotation of the rolls are also important in producing a high reduction without slippage. More specifically, in order to hot roll copper and brass strand with a large bite (e.g. in excess of 1 inch) and a low bite ratio (e.g. 7:1), the drive system for the working rolls must have a comparatively high torque (typically in excess of 1,000 ft-lbs.) to achieve a large bite and a comparatively low speed (typically less than 400 rpm) to couple the rolling mill speed to that of the caster. In addition, it is necessary to vary these parameters depending on the particular product being run and other factors such as the roll diameter. Heretofore mill stands requiring a variable high torque at a low speed have used electric motors with a reducing gear train. This arrangement provides the required operating characteristics, but it is a costly system that takes up a comparatively large amount of space that dominates the mill itself.
Torque requirements are also interrelated with other design factors such as roll diameter and anticipated separation forces and thus, size and cost of the mill, the gauge of the rolled product and friction. Heretofore the lower rolling force of small diameter rolls and their ability to roll thin gauge products has generally been balanced against offsetting considerations through the use of back-up rolls. If two rolls are used where substantial separation forces are produced, prior art mills have used costly, massive rolls with very large diameters, e.g. two to three feet. Heretofore, high reduction rolling using unsupported (two high) small diameter rolls (less than at least one foot in diameter) has not been commercially practical. U.S. Pat. No. 4,218,907 describes a two high mill which provides operating characteristics only achievable previously with four high mills. However, this two high mill requires a special bearing assembly which supports the rolls over all or most of their length.
Other design problems arise from the fact that the material being rolled is hot. One well known problem is that the hot strip product will heat the working rolls and cause their contours to change due to thermal expansion. This in turn can cause changes in the gauge and profile of the rolled strip. U.S. Pat. No. 4,262,511 describes as a solution a system that distributes a coolant over the rolls in response to a signal indicative of the shape of the rolled product. Other systems simply adjust the mill gap to accommodate for temperature changes. In cold rolling, profiled rolls have been used. No known systems, however, have produced a strip product having consistently uniform dimensions where the input to the mill is a very hot strand that is not already in a strip form. When hot rolling thin gauge materials with a high thermal conductivity, it is also important not to quench the material with the rolls while at the same time not overheating the strip or the rolls from an input of mechanical energy.
Another problem in rolling operations, particularly hot rolling operations on a continuous strand/strip product, is that the strand or strip usually develops or is subjected to significant forces that tend to move the product laterally with respect to the rolls. Various mechanical guides and restraints are known. However, none have proven to be highly effective in controlling this problem because the mechanical guide or restraint is spaced from the point where the rolls engage the product. This spacing allows the product to move in response to the forces once it passes the guide or restraint. Also, many guides or restraints do not closely control the lateral position of the strands even at the guide member.
It is therefore a principal object of this invention to provide a roll for a metal, particularly a hot rolling mill that rolls a continuously cast hot metallic strand into a narrow strip, where the rolls are designed to self-center the strand or strip on the working portion of the roll.
Another major object of this invention is to provide a hot rolling mill utilizing such rolls where the mill is only two high and uses small diameter rolls yet is capable of producing a high reduction with precisely controlled dimensions and profile of the rolled strip product.
A further object of this invention is to provide a hot rolling mill utilizing rolls with the foregoing advantages that also accommodate for the thermal expansion caused by working a hot material.